The Moon's surface is thought to be covered almost everywhere by a layer of regolith. Regolith is a term meaning soil produced by weathering of local rock. On the Moon weathering is mostly caused by impacts, both large and small. In fact the smaller impacts, micrometeorite impacts, produce most of the upper portion of the regolith. An abundance of super fine and unconsolidated grains are produced by the astounding number of micro-impacts that have occurred over the past 4 billion years. In addition to rocks breaking apart into finer and finer fractions, over time space weathering makes rocks darker and redder. This effect is especially noticeable around steep surfaces where unweathered material slides down and stands in high contrast with its surroundings.

Medium resolution view of the full field of view (2.2 kilometers) showing Rima Marius as seen in LROC NAC observation M137882287R [NASA/GSFC/Arizona State University].

Today's Featured Image focuses on a portion of southern bank of Rima Marius. Dark material, likely mature regolith, shows forked shapes at the edge of the mare. How were these irregular patterns formed? They could be remnants of collapsing plateau edges, which exposes fresh and bright slope surface in between the dark remnants. Or it might be flow marks of darker materials. If these dark fingers are flows how did they form? Perhaps small moonquakes destablized mature regolith, which then slid a bit down the slopes of the rille?

LROC Wide Angle Camera (WAC) 100 meter-per-pixel monochrome mosaic of the vicinity of Rima Marius. The blue box and while arrow indicate locations of full NAC frame and the LROC Featured Image released May 17, 2011. View full-sized WAC context frame HERE [NASA/GSFC/Arizona State University].